In two new studies published this week, scientists have refined their understanding of the SARS virus and identified more animal species that can carry the disease.

Wild animals are thought to have carried the disease before humans, but it hasn't been clear which species. Now, researchers have added ferrets and cats to the list of animals susceptible to the SARS virus.

Previously, researchers had found a SARS-like virus in masked palm civets, raccoon dogs, and Chinese ferret badgers from a Chinese market. And there were infected cats at the Amoy apartments in Hong Kong, China, where 100 people had SARS last year.

The new research demonstrates that ferrets and cats could be useful models to study the disease in the laboratory. Albert D.M.E. Osterhaus of Erasmus Medical Centre in the Netherlands led the study, which was published in Nature.

The scientists infected cats and ferrets with a virus isolated from a SARS-patient. The cats showed no outward signs of infection, but both ferrets and cats were infected with the virus. In addition, infected cats and ferrets were able to transmit the disease to non-infected animals in cages.

In another study, researchers have further detailed the structure of an enzyme in the SARS virus that, if disabled by drugs, could kill the virus. The enzyme, called the main viral protease, helps the virus replicate inside cells.

They created a three-dimensional crystal structure of the enzyme, and discovered that it changes form at varying levels of acidity and that it binds to inhibitors differently than other coronaviruses. The study was led by Zihe Rao of the Chinese Academy of Science in Beijing.

The research refines a theoretical model of the enzyme that was developed last spring by a group of German researchers.

The researchers say the study could improve the search for drugs to target the SARS virus, and they are in talks with Pfizer.The company says it is testing a number of compounds against the SARS virus, including some in development for the common cold.

“Since the disease hit hardest in China, I really think we have an obligation to do these type of studies,” says Rao.

Yang, H. et al. The crystal structures of severe acute respiratory syndrome virus main protease and its complex with inhibitor. Proceedings of the National Academy of Sciences USA Published online October 29, 2003.